1. Field of the Invention
The present invention relates to a cermet tool capable of, for example, cutting steel at high speed, and to a method for manufacturing the same.
2. Description of the Related Art
Conventionally, as described in, for example, Japanese Patent Application Laid-Open (kokai) No. 9-104943, a cermet tool is used for cutting steel. Cermet is a sintered hard alloy made of a bonding-phase metal of the iron group, such as Ni, and a hard-phase material, such as TiCN.
A sintered skin which has been formed during sintering may remain on the surface of a cermet tool of this type. Conventionally, exudation of a bonding-phase metal, such as Ni, into the sintered skin is observed. In the case where a bonding-phase metal has exuded into the sintered skin, the exuded bonding-phase metal and a workpiece material often fuse together, thereby imposing excessive cutting resistance on the cutting edge of the tool with resultant chipping of the cutting edge.
To cope with this problem, as shown in FIG. 8, a hard layer P2 harder than an interior portion P1 of a cermet tool is formed on the surface of the tool (the composition of the hard layer P2 differs from that of the interior portion P1).
The amount of a bonding-phase metal contained in the hard layer P2 is less than that contained in the interior portion P1 of the tool (i.e., the hard layer P2 has a higher hard-phase material content). Through formation of such a hard layer P2 on the surface of the tool, resistance of the tool to wear and chipping is improved.
By employing the hard layer P2, wear resistance is improved. Also, by reducing the amount of the bonding-phase metal contained in the hard layer P2 (the bonding-phase metal potentially causes fusion), fusion is prevented, to thereby improve resistance to chipping.
The above-described prior art techniques can improve resistance to wear and chipping to a certain extent, but not to a sufficient extent.
Specifically, since the coefficient of thermal expansion of the bonding-phase metal, such as Ni, differs significantly from that of the hard-phase material, such as TiCN, contained in a large amount in the hard layer P2, the cutting edge of the tool tends to chip during machining which involves great temperature variations and a heavy thermal load.
The present invention has been achieved in view of the above problems. It is therefore an object of the present invention to provide a cermet tool having excellent resistance to chipping and wear as well as to provide a method for manufacturing the same.
Accordingly, the present invention provides a cermet tool comprising a hard layer formed on a surface thereof, characterized in that an exposure portion is formed at a nose so as to expose an interior portion of the tool and in that the hard layer is disposed around the exposure portion.
According to the present invention, as shown in FIG. 1, the exposure portion where an interior portion of the cermet tool is exposed is formed at the nose of the tool (through, for example, radiusing), and the hard layer is disposed around the radiused portion. FIG. 1 shows a cermet tool assuming substantially the form of a rectangular parallelepiped (negative tip) as viewed from above the rake face, in which the hard layer located on the rake-face side is removed.
Accordingly, when a workpiece is to be machined by the nose of the cermet tool, the exposure portion, which is softer than the hard layer and has toughness, is brought into contact with the workpiece. Therefore, even when a large force is imposed on the nose, the nose is less likely to chip. The exposure portion is surrounded by the hard layer, which is harder than the interior portion of the tool. Therefore, even when cutting chips hit the hard layer during cutting, the hard layer does not wear significantly.
Since the cermet tool of the present invention has an exposure portion, where the interior portion of the tool is exposed, and a hard layer disposed around the radiused portion, the tool exhibits excellent resistance to chipping and wear.
In the present invention, the term xe2x80x9ccermetxe2x80x9d means a sintered hard alloy comprising at least a hard-phase material such as TiCN, and a bonding-phase metal of the iron family such as Ni; and the term xe2x80x9ccermet toolxe2x80x9d means a tool formed of such an alloy.
Preferably, the exposure portion is a radiused portion or rather circular arc or a chamfered portion formed such that an interior portion of the tool is exposed. Preferably the exposure portion is formed at a corner portion (nose) of the tool.
Preferably, the radius of the radiused portion is 0.04 mm to 0.16 mm. By employing a specified radius, the radiused portion can be exposed appropriately by polishing the nose. Thus, the nose is less likely to chip during cutting, thereby enabling favorable cutting.
Preferably, the thickness of the hard layer is 2 xcexcm to 20 xcexcm. By employing the specified thickness, the hard layer is appropriately disposed around the radiused portion when the radiused portion is to be exposed by polishing the nose. Thus, the nose becomes less likely to wear during cutting, thereby enabling favorable cutting.
When the thickness of the hard layer is not less than 2 xcexcm, the hard layer provides a sufficient wear resistance effect. When the thickness of the hard layer is not greater than 20 xcexcm, the radiused portion can be easily exposed by radiusing in a conventionally employed range.
Particularly, when the radius of the radiused portion is 0.04 mm to 0.16 mm and the thickness of the hard layer is 2 xcexcm to 20 xcexcm, the radiused portion can be readily formed at the nose by radiusing such that the interior portion of the tool is exposed to an appropriate extent.
Advantageously, the tool contains as a bonding phase at least two iron-group metals and contains as a hard phase at least two members selected from the group consisting of carbides, nitrides, and carbonitrides of elements in Groups 4A, 5A and 6A of the periodic table.
As schematically shown in FIG. 2, a cermet tool comprises a hard phase which contains hard grains, such as TiCN; a bonding phase composed of bonding-phase metals and a binder; and a solid solution composed of bonding-phase and hard-phase components. The present invention specifies advantageous compositions of the cermet tool.
Examples of the iron-group metals include Ni, Co, and Fe.
Examples of carbides, nitrides, and carbonitrides of elements in Groups 4A, 5A, and 6A include TiC, which is a carbide of a Group 4A element; TiN, which is a nitride of a Group 4A element; TiCN, which is a carbonitride of a Group 4A element; VC, NbC and TaC, which are carbides of Group 5A elements; and Mo2C and WC, which are carbides of Group 6A elements.
Preferably, substantially no exudation of bonding-phase metals into the sintered skin of the cermet tool is observed.
When no exudation of bonding-phase metals into the sintered skin of the cermet tool is observed, the cutting edge of the tool and a workpiece material are less likely to fuse together, thereby preventing chipping of the cutting edge, which would otherwise result from fusion.
A degree which corresponds to substantially no exudation of bonding-phase metals being observed may be equivalent, for example, to a bonding-phase-metal content of not greater than 1.9% by weight as measured by means of an SEM (an energy dispersive X-ray analyzer attached to a scanning electron microscope). Since measurement of a bonding-phase-metal content by means of the SEM covers not only the surface of the sintered skin but also a small distance into the interior portion of the skin, the bonding-phase metal content of not greater than 1.9% by weight may be considered to indicate substantially no exudation of bonding-phase metals being observed.
Preferably, when the amount of bonding-phase metals as measured substantially at a center of the interior portion of the tool is taken as 100% by weight, the amount of bonding-phase metals contained in the hard layer is not greater than 11% by weight. Accordingly, the hard layer can be defined by the amount of bonding-phase metals contained in the hard layer.
When the bonding-phase-metal content of the hard layer is not greater than 11% by weight, the amount of bonding-phase metals which are present in the vicinity of the surface of the sintered skin and which are potentially exuded into the sintered skin is small. Thus, fusion is less likely to occur, thereby imparting excellent resistance to chipping.
When bonding-phase metals are contained in excess of 11% by weight, the cutting edge of the tool will chip due to fusion. This has been experimentally confirmed, as described below. When the hard layer of such a composition has a thickness of 2 xcexcm to 20 xcexcm, excellent wear resistance is achieved.
xe2x80x9cA center of the interior portion of the toolxe2x80x9d is a position where the amount of bonding-phase metals is measured for use as a reference in compositional comparison, since variation in composition at such a portion is small.
Preferably, a sintered skin is present on a flank and/or rake face of the cermet tool.
When the sintered skin remains on the surface of the tool; for example, when the surface of the hard layer retains a sintered skin formed during sintering, application of a surface treatment, such as coating, to the tool can be omitted, thereby effecting an advantage in terms of cost.
The invention also provides a method for manufacturing a cermet tool, characterized as comprising: sintering a cermet material compact; and lowering the temperature of a resultant sintered body under a reduced pressure, preferably in the range of from 5 Torr to 50 Torr, in a nitrogen atmosphere for 15 to 25 minutes.
In the present specification, a pressure of 1 Torr is understood to be approximately 130 Pa.
According to the manufacturing method, when the hard layer is to be formed on the surface of the tool, the amount of bonding-phase metals contained in the hard layer can be reduced, for example, to not greater than 11% by weight, to thereby suppress exudation of bonding-phase metals into the sintered skin.
The method of the present invention specifies nitrogen as an atmospheric gas for use in cooling, for the following reason. By introducing nitrogen gas, for example, 4A-, 5A- and 6A-group components of the surface of the tool are nitrided to thereby form a nitriding phase composed of nitrides of 4A-, 5A- and 6A-group components. The nitriding phase shows poor wettability to bonding-phase metal components, so that exudation of bonding-phase metals into the surface of a sintered body is suppressed.
A temperature-lowering time of 15 to 25 minutes is employed for the following reason. When the temperature-lowering time is less than 15 minutes, the effect of suppressing exudation of bonding-phase metals becomes insufficient. When the temperature-lowering time is in excess of 25 minutes, a graphite phase is formed.
If the compact is sintered at a temperature of 1400xc2x0 C. to 1600xc2x0 C. in a pressure-reduced atmosphere (for example, in an inert gas other than nitrogen), sufficient sintering occurs.
Preferably, the method for manufacturing a cermet tool is further characterized in that the partial pressure of nitrogen in the pressure-reduced nitrogen atmosphere is 5 Torr to 50 Torr.
The partial pressure of nitrogen is limited to 5 Torr to 50 Torr for the following reason. When the partial pressure of nitrogen is not higher than 5 Torr, the effect of suppressing exudation of bonding-phase metals becomes insufficient. When the partial pressure of nitrogen is not lower than 50 Torr, excess carbon generated through nitriding, for example, of 4A-, 5A- and 6A-group components forms a graphite phase, causing an impairment in physical properties (such as strength).